Improvement of Cycle Capability of FeS₂Positive Electrode by Forming Composites with Li₂S for Ambient Temperature Lithium Batteries

FeS 2 -Li 2 S composite positive electrode material, consisted of mainly Li 3 Fe 2 S 4 and Li 2.33 Fe 0.67 S 2 , was prepared for improving the cycle capability of FeS 2 in rechargeable lithium batteries with liquid electrolytes operated at ambient temperature. The electrochemical tests demonstrated that the FeS 2 -Li 2 S composite cells showed a relatively higher capacity retention after 15 cycles ( ca. 64%) as compared with that of the FeS 2 cells ( ca. 26%), though both sample cells showed comparable initial discharge capacities ( ca. 740 and 790 mAh·g −1 (4.2 and 4.5 mAh·cm −2 ), respectively). The improved cycle capability of the FeS 2 -Li 2 S cells was ascribed particularly to rather unchanged discharge plateau at ca. 2.0 V, which was reported to correspond to the reduction of sulfur. The ex-situ S K-edge X-ray absorption fine structure measurements indicated that the redox reaction of sulfur ions (S 2 2− + 2 e − ⇔ 2S 2− ), associated with Li extraction / insertion reactions, was still active after 15 cycles in the FeS 2 -Li 2 S electrode, while such redox reactions almost disappeared in the FeS 2 electrode. The atomic displacements required in the initial delithiation / lithiation were reduced in the FeS 2 -Li 2 S composite as compared with FeS 2 , which would give rise to the suppression of the structural irreversibility in the following charge / discharge, resulting in the improved cycle capability.